Martindale vt28 voltage tester and pd690 proving device martindale vt28pd safely proving a circuit to be dead requires the use of a voltage indicator and a known live source to test the indicator both prior to and after checking for the presence of voltage the combination of the martindale vt28 voltage indicator and pd690 proving unit allows you to do just that ensuring that you always have a known source to test againstthe vt28 is a two pole voltage indicator with bright led indicators